This application claims the priority of U.K. Patent Application No. GB 0105547.4, filed Mar. 7, 2001 and U.K. Patent Application No. GB 0125749.2, filed Oct. 26, 2001, the contents of which are incorporated herein by reference.
1. Field of the Invention
This invention relates to electric hammers having an air cushion hammering mechanism.
2. Description of the Related Art
Such hammers will normally have a housing and a hollow cylindrical spindle mounted in the housing. The spindle allows insertion of the shank of a tool or bit, for example a drill bit or a chisel bit, into the front end thereof so that it is retained in the front end of the spindle with a degree of axial movement. The spindle may be a single cylindrical part or may be made of two or more cylindrical parts, which together form the hammer spindle. For example, a front part of the spindle may be formed as a separate tool holder body for retaining the tool or bit. The hammer is normally provided with an impact mechanism comprising a motor that drives a piston, which may be a hollow piston, to reciprocate within the spindle. The piston reciprocatingly drives a ram by means of an air cushion located between the piston and the ram. The impacts from the ram are transmitted to the tool or bit of the hammer via a beatpiece located within the spindle.
Some hammers can be employed in combination impact and drilling mode in which the spindle, or a forwardmost part of the spindle, and hence the bit inserted therein, will be caused to rotate at the same time as the bit is struck by the beat piece.
When the hammer is to be used the forward end of a tool or bit is pressed against a workpiece, which urges the tool or bit rearwardly within the hammer spindle. The tool or bit in turn urges the beatpiece rearwardly into its operating position in which the rearward end of the beatpiece is located within the reciprocating path of the ram. In the operating position the beatpiece receives repeated impacts from the ram. When the hammer is in use, the forward impact from the ram is transmitted through the beatpiece to the bit or tool and through the bit or tool to the workpiece. A reflected impact is reflected from the workpiece and is transmitted through the bit or tool to the beatpiece. This reflected, or reverse impact must be absorbed within the structure of the hammer in such a way that the reverse impacts do not over time destroy the hammer and so that the reverse impacts are not transmitted to the end user.
When the user takes the tool or bit of the hammer away from the workpiece, the next forward impact of the ram on the beatpiece urges the beatpiece forwardly into its idle mode position. The beatpiece can move forwardly and stay forwardly because the tool or bit is no longer urging it rearwardly, because the tool or bit also assumes a forward idle mode position. Because the beatpiece does not now offer much resistive force against the ram, the ram also moves into a forward idle mode position. In the idle mode position of the ram, the air cushion is vented and so any further reciprocation of the piston has no effect on the ram. This forward movement of the components on entry into idle mode generates the greatest impact forces on the structure of the hammer, in particular on the hammer spindle. This is because the forward impact force of these parts on entry into idle mode is not transferred to the workpiece, but has to be absorbed by structure of the hammer itself. Thus, the number of idle strikes, ie. the number of reciprocations of the ram, beatpiece and tool or bit, when the bit or tool is removed from the workpiece need to be minimised in order to minimise the number of high impact force idle strikes that have to be absorbed by the structure of the hammer. This is generally achieved by catching the ram and/or the beatpiece in their idle mode positions so that they cannot slip rearwardly to cause the ram to move into a position in which the air cushion is closed and the ram and thus the beatpiece begin to reciprocate again.
In order for the maximum impact to be transmitted from the ram to the tool or bit, via the beatpiece, the beatpiece must be co-axial with the spindle. Thus, high efficiency is achieved if the reciprocating movement of the beatpiece within the spindle is guided to ensure good axial alignment with the axis of the spindle.
Hammers are necessarily operated in very dusty and dirty environments. If dust gets into the spindle of the hammer it can cause abrasion between the reciprocating parts and, in particular, can cause seals between the ram and the spindle to become worn. Wearing of the seal around the ram will cause the air cushion to deteriorate, which will eventually lead to impacts occurring between the beatpiece and the ram which can seriously damage the hammer. Therefore, a further issue in the longevity of the working life of a hammer is its sealing against dust. The reciprocation of parts within the spindle can draw dust rearwardly inside the hammer spindle, where damage can be caused.
Attempts to solve these problems have been made and examples of the resulting hammer arrangements are known from U.S. Pat. No. 4,476,941 and DE196 21 610.
The arrangement in U.S. Pat. No. 4,476,941 has a complicated multi-part spindle arrangement with a first sleeve for guiding a rearward reduced diameter portion of the beatpiece, which sleeve extends from the inside to the outside of the spindle, between two spindle parts. The impact of the beatpiece on entry into idle mode is absorbed by a second sleeve, located forward of the first and within a different one of the spindle parts. The second sleeve also guides a forward increased diameter portion of the beatpiece. The arrangement in U.S. Pat. No. 4,476,941 has a problem with dust ingress, in particular during periods when a tool or bit is removed from the tool holder of the hammer, into the portion of the forward sleeve where the beatpiece is guided. This problem is exacerbated by the pumping nature of the increased diameter portion of the beatpiece which is guided within the second sleeve. The small amount of axial support for the first sleeve which is mounted between spindle parts, along with usual tolerance limitations for component parts could lead to a reduced accuracy of axial guiding of the beatpiece by the sleeves. The design in U.S. Pat. No. 4,476,941 results in a complicated multi-part spindle, beatpiece guiding and damping structure, with the associated assembly problems and cost implications.
The arrangement in DE196 21 610 overcomes some of the problems discussed above, but still has the disadvantage of a relatively complex three part spindle arrangement, having sleeves for beatpiece guiding mounted and guided in different spindle parts. Again the usual tolerance issues between spindle parts can reduce the accuracy with which the beatpiece is guided and complicates the sealing of dust from the inside of the spindle. Again the design in DE196 21 610 has a complicated multi-part spindle, beatpiece guiding and damping structure, with the associated assembly problems and cost implications.
The present invention aims to provide a beatpiece guiding and damping arrangement which solves all of the problems discussed and which results in a relatively simple and easy to assemble structure.
According to the present invention there is provided an electrically powered hammer comprising:
a hollow spindle having a reduced diameter tool holder portion at its forward end in which a tool or bit can be releaseably mounted for limited reciprocation, within which spindle is reciprocatingly mounted a piston and a ram of an air cushion hammering mechanism;
a beatpiece having an increased diameter mid-portion, which beatpiece is located within the spindle between the ram and the tool or bit for transmitting repeated impacts from the ram to the tool or bit; and
a two part sleeve arrangement located within the spindle and having an increased internal diameter mid-portion for receiving the increased diameter portion of the beatpiece and a reduced internal diameter forward and rearward portion for guiding the forward and rearward ends respectively of the beatpiece in all working positions of the beatpiece,
characterised in that the sleeve arrangement is formed by a forward sleeve and a rearward sleeve which are both guided with tight radial tolerances and with a slight axial play within and by the same one piece spindle part and in which the forward axial movement of the forward sleeve is limited by a reduced internal diameter portion of the spindle and the forward axial movement of the rearward sleeve is limited by the forward sleeve.
The sleeve arrangement according to the present invention enables easy assembly of the sleeves and beatpiece and associated components, as a sub-assembly, within a single spindle component part. It also enables simple sealing of the inside of the spindle from dust, as the sleeve arrangement itself forms an effective barrier to dust ingress. In addition, the sleeve arrangement facilitates a reduction in the intensity of impacts on the structure of the hammer on entry into idle mode and catching of the ram and beatpiece in idle mode. On entry into idle mode, the increased diameter portion of the beatpiece hits the forward sleeve and imparts forward momentum to the forward sleeve and itself moves rearwardly, but with relatively low momentum thus facilitating catching of the beatpiece and/or ram. Due to the slight axial play of the sleeve arrangement within the spindle, on entry into idle mode a small gap is located or generated between the front of the forward sleeve and the reduced internal diameter portion of the spindle. When the beatpiece hits the forward sleeve the forward sleeve moves forwardly to close the gap and impact the reduced diameter portion of the spindle. The reflected impact from this collision of the front sleeve causes the front sleeve to then move rearwardly, but not with sufficient speed to impact the beatpiece. Instead the rearward momentum for the forward sleeve is absorbed by a collision with the rearward sleeve, and can be transmitted thereby to the spindle. Thus, the only a small part of the reflected impact from collisions taking place within the spindle on entry into idle mode is transmitted to the beatpiece. As will be described below, the two part sleeve arrangement enables additional advantages to be achieved in an easy to assemble sub-assembly.
As well as the sleeve arrangement itself forming a barrier to the ingress of dust into the interior of the spindle, an annular seal can be located between the beatpiece and said one piece spindle part, in front of the sleeve arrangement. Alternatively, an annular seal can be located between the beatpiece and the forward end of the forward sleeve, and this seal may be recessed within the forward end of the forward sleeve. In this way the beatpiece is guided within the sleeve arrangement in a dust free region of the spindle. In addition an annular seal can be located between the forward sleeve and said one piece spindle part. Thus, the arrangement according to the present invention enables the interior of the spindle to be effectively sealed from the ingress of dust by simple annular seals, such as rubber O-ring seals.
The sleeve arrangement can be arranged to enclose the mid-portion of the beatpiece to form a self-contained sub-assembly, which is assembled into said one piece spindle part. This provides a simple assembly procedure.
In order to reliably catch the beatpiece and/or ram in its forward position on entry into idle mode the mass of the front sleeve preferably is less than or equal to the mass of the beatpiece. In a preferred embodiment the mass of the front sleeve is less than half of the mass of the beatpiece.
In one preferred embodiment of the present invention the beatpiece has a second increased diameter portion, rearward of the first, which second portion is engageable with a resilient beatpiece catching ring. The ring is mounted preferably within the rearward sleeve and is arranged to catch the beatpiece in a forward position in idle mode, by limiting the rearward movement of the second increased diameter portion during idle mode. The inclusion of the beatpiece catching arrangement in the sleeve arrangement, again simplifies assembly as the beatpiece catching ring can be pre-assembled in a sleeve arrangement sub-assembly, which sub-assembly is then assembled into said one piece spindle part.
An annular gap is formed between the peripheral surface of the increased external diameter portion of the beatpiece and increased internal diameter portion of the sleeve arrangement. Thus, as the beatpiece reciprocates within the sleeve arrangement, grease is free to move around the increased diameter portion of the beatpiece and the reciprocation of the beatpiece is less likely to cause dust to travel rearwardly along the spindle.
In one preferred embodiment of the present invention a metal beatpiece impact ring is mounted in the rearward sleeve behind the rearward facing surface of the increased diameter portion of the beatpiece for absorbing reverse impacts from the beatpiece and transmitting the impacts to the rearward sleeve during normal use of the hammer. This enables efficient transmission of reverse impacts from the beatpiece during normal operation of the hammer. Again the impact ring can be assembled into a sleeve arrangement sub-assembly, before assembly of the sub-assembly into said one piece spindle part, thus facilitating efficient assembly procedures. In addition, a damping ring may be mounted in the rearward sleeve behind the impact ring for damping the impacts transmitted from the impact ring to the rearward sleeve. Preferably, the beatpiece damping ring and the beatpiece catching ring are formed by the same component.
The reverse impacts from the beatpiece in normal use of the hammer are efficiently transmitted from the increased diameter portion of the hammer to the spindle via the rearward sleeve. In one preferred embodiment a resilient O-ring is located between a rearward facing external shoulder of the rearward sleeve and a fixing for axially limiting the rearward movement of the rearward sleeve within said one piece spindle part and during normal operation of the hammer, the increased diameter portion of the beatpiece repeatedly abuts a forward facing internal shoulder of the rearward sleeve. Thus, the reverse impacts are transmitted from the beatpiece to the rearward sleeve and are then damped by the O-ring before being transmitted to the spindle via the fixing. Thus, the intensity of the reverse impacts from the beatpiece which are transmitted to the spindle is reduced.
In one preferred embodiment of the present invention a resilient O-ring located between a first forward facing shoulder of the forward sleeve and a first rearward facing shoulder of the spindle urges the forward sleeve into a rearward position within the spindle to open up a gap between a forward facing part of the forward sleeve and a rearward facing part of the spindle, which gap is closed by forward movement of the sleeve on entry into idle mode of the hammer.
The present invention enables simplification of the spindle structure and the hollow spindle may be formed as a single component. Alternatively, the hollow spindle may be formed as two components, for example when it is desired to remove and/or rotate a forward tool holder portion of the spindle from and/or with respect to a rearward portion of the spindle. In particular, a first spindle component may house the piston, ram and beatpiece and a second component may form a tool holder which is removable from the first component.